Here are some anecdotal statistics for this site. During the month of November
2014, snarfed.org received 796,990 POSTs to its comment
wp-comments-post.php, vs just 2139 POSTs to
xmlrpc.php (based on
server logs). Of those, 9 comments and 3 trackbacks were ham. I also use XML-RPC
to create and edit posts. So there was at least ~400x more comment form spam
than pingback/trackback spam during that month.
Also posted on indiewebcamp.com/spam.